Optical Constants of Organic Tholins Produced in a Simulated Titanian Atmosphere" from Soft X-Ray to Microwave Frequencies
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ICARUS 60, 127--137 (1984) Optical Constants of Organic Tholins Produced in a Simulated Titanian Atmosphere" From Soft X-Ray to Microwave Frequencies B. N. KHARE AND CARL SAGAN Laboratory for Planetary Studies, Cornell University, Ithaca, New York 14853 E. T. ARAKAWA, F. SUITS, T. A. CALLCOTT, AND M. W. WILLIAMS Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 Received January 20, 1984; revised May 23, 1984 As part of a continuing series of experiments on the production of dark reddish organic solids, called tholins, by irradiation of cosmically abundant reducing gases, the synthesis from a simulated Titanian atmosphere of a tholin with a visible reflection spectrum similar to that of the high altitude aerosols responsible for the albedo and reddish color of Titan has been reported (C. Sagan and B. N. Khare, 1981, Bull. Amer. Astron. Soc. 13, 701 ; 1982, Orig. Life. 12, 280) and [C. Sagan, B. N. Khare, and J. Lewis, in press. In Saturn (M. S. Matthews and T. Gehrels, Eds.), Univ. of Arizona Press, Tucson]. The determination of the real (n) and imaginary (k) parts of the complex refractive index of thin films of such tholin prepared by continuous D.C. discharge through a 0.9 NJ0.1 CH4 gas mixture at 0.2 mb are reported. For 250 ,~ -< h -< 1000/zm, n and k have been determined from a combination of transmittance, specular reflectance, interferometric, Brewster angle, and ellipsometric polarization measurements; experimental uncertainties in n are estimated to be -+0.05, and in k -+ 30%. Values of n (=1.65) and k (=0.004 to 0.08) in the visible range are consistent with deductions made by ground-based and spacecraft observations of Titan. Maximum values of k (=0.8) are near 1000 .~, and minimum values (=4 × 10 41) are near 1.5 /xm. Many infrared absorption features are present in k(h), including the 4.6-/~m nitrile band. © 1984 Academic Press, Inc. INTRODUCTION solids, called tholins, are produced by the irradiation of a wide variety of cosmically The Voyager 1 discovery (or confirma- abundant reducing gases (Sagan and Khare, tion) of nine simple organic molecules as 1979). The tholin produced at Cornell ex- minor constituents of the atmosphere of Ti- hibits a visible spectrum short of the onset tan (Hanel et al., 1981; Maguire et al., 1981 ; at 0.63 /zm of the CH4 Kuiper bands very Kunde et al., 1981), surmounting an unbro- similar to that observed for the Titan aero- ken dark reddish haze layer (Smith et al., sols (Khare et al., 1981; Sagan et al., 1984). 1981), tends to confirm earlier suggestions Gas chromatography/mass spectrometry of of an organic aerosol on Titan produced in pyrolyzates prepared from this tholin re- situ from the major atmospheric constitu- veals over 75 separate compounds, includ- ents (Sagan, 1971, 1973, 1974; Khare and ing many molecules of fundamental biologi- Sagan, 1973). In experiments previously re- cal significance on Earth (Khare et al., ported, we have exposed a simulated Tita- 1982, 1983). nian atmosphere [0.09 CHd0.91 N2 at 73 A determination of the complex refrac- mb] to high-frequency electrical discharge tive index of various kinds of simulated Ti- for 4 months and have produced a dark red tan tholins over as wide a frequency range solid that we propose is similar to the ob- as possible would permit comparison with served Titan aerosol (Sagan and Khare, refractive index estimates made at visible 1981, 1982; Sagan et al., 1984). Similar wavelengths from ground-based and space- 127 0019-1035/84 $3.00 Copyright © 1984 by AcademicPress, Inc. All rights of reproductionin any form reserved. 128 KHARE ET AL. craft photometric and polarimetric observa- atmosphere (Smith et al., 1982). Cosmic tions; would allow calculations on the rays and, to a lesser extent, Saturn magne- transmissivity of the Titan atmosphere to tospheric electrons deposit most of their sunlight, and therefore on the validity of energy in the Titanian lower atmosphere, at greenhouse mechanisms to explain the an altitude still inaccessible to observation small increment (-~13°C) of Titan surface (Sagan and Thompson, 1984). Thus the tho- temperature over Titan equilibrium temper- lins produced in the present study are likely ature; would enable modeling of observed to correspond to the reddish aerosol haze in albedo/color variations both in space and in the upper atmosphere of Titan. In the re- time in the aerosols of Titan; and might be gion simulated, at the base of the Titanian useful for calculations of radiative transfer thermosphere, the ambient temperatures in the infrared and microwave regimes. are =I70°K. The laboratory experiments Knowledge of the optical properties is also were carried out at room temperature, but an essential prerequisite for putting on a because the difference in effective tempera- firmer basis the idea that Titan tholin aero- ture between the energy source for molecu- sols are in fact the major chromophore in lar dissociation and the ambient tempera- the atmosphere of Titan. ture is so large, both in the laboratory and Reliable determination of the real part, n, on Titan, the choice of laboratory ambient and the imaginary part, k, of the complex temperatures may not affect the results sig- refractive index of the tholin by measuring nificantly. the reflectivity of optically thick samples A Technics Hummer V film deposition (pressed pellets) and then performing a chamber at Oak Ridge National Laboratory Kramers-Kronig analysis proved ex- was employed. The experimental setup is tremely difficult, because the surface of the shown in Fig. 1, and the details of the elec- samples were not truly specular. It became trode and field configurations are given in evident that reliable determinations of n Fig. 2. The apparatus consists essentially of and k would require transmission measure- two horizontal aluminum electrodes of di- ments, which in turn would require the ameter -~7.6 cm, separated by a vertical preparation of optically thin films of Titan distance of -~5.7 cm, enclosed coaxially tholin. within a vertical glass cylinder of internal Thin films of tholin were generated from diameter =10 cm. Clean glass or alkali a gas mixture of 0.9 N2 and 0.1 CH4 by vol- halide substrates for the tholin films were ume at a total pressure of 0.2 mb. This cor- placed in a nearly vertical position on the responds to a radial distance from the cen- lower electrode and leaning against the in- ter of Titan of about 2825 km, just at the top side wall of the cylindrical glass reaction of the main cloud deck viewed by Voyagers vessel. The N2 used was prepared from liq- l and 2, and below most of the visible aero- uid nitrogen of minimum purity 99.997%, sol haze. The solar ultraviolet flux at X < containing <3 ppm H20 and <5 ppm 02; 900 ,~, Saturn magnetospheric electrons the CH4 was chemically pure (CP) grade, and protons, solar wind electrons, and cos- 99% purity. A typical analysis of CP grade mic rays are all able to break N2 chemical methane revealed 0.6% N2, 0.5% 02, 0.2% bonds and synthesize nitrogenous organics CO and CO2, and 0.13% C2H4 as impurities. from NJCH4 atmospheres. Saturn magne- Both gases were obtained from Linde Divi- tospheric particles and interplanetary elec- sion, Union Carbide Corporation. The gas trons deposit most of their energy at radial mixture continuously flowed through the l- distances between 3200 and 3600 km from liter chamber at a rate -~0.05 cc/sec. A 15- the center of Titan (Sagan and Thompson, mA direct current electrical discharge was 1984), a region corresponding to an ob- maintained by a 200-V potential difference served extreme uv opacity in the Titanian between the electrodes. The tholins pre- OPTICAL CONSTANTS OF ORGANIC THOLINS 129 IONIZATION • CHAMBER 15ma M'~ots~o'~ ~,~scs. ---2OO V DC. METHANE VALVE ~l'-~ ~~ PRESSUREGAUGE VALVE ~I~LVE NITROGEN t i! -- TANK GA a-.... ;I :zz:cC~" FOR OU "3{'/ MIXED GASES ~ PU~ PUMP TO NEEDLE VALVE Fi~. 1. Experimental apparatus, incorporating Technics "Hummer V" film deposition chamber, used in preparing thin film of Titan tholin on various substrates. sumably form by quenching and reaction of and the other being air or vacuum, is given the ionization and dissociation products in by Hall and Ferguson (1955). We consider a the region of the discharge, and then diffuse wavelength region in which a thin homoge- outward, where they are deposited on the neous tholin film characterized by a thick- substrates. Most of the films were formed ness, t, and a real part, n, of the complex on glass microscope slides, but some were refractive index is transparent, interference deposited on disks of CaF2, LiF, or CsI. fringes are seen, and the normal incidence Since the substrates were of different sizes, transmittance T at wavelength X is the geometry for film formation varied. The T = [1 + 4R2(1 - Rf2) 2 sin 2 (27rnt/h)]-J, (1) thickness at each point of a given film de- pends on this geometry, on the deposition where gf is the reflectance of the tholin time and on the time-variable configuration film. Maxima in T occur at integer values of of the discharge. Thus the thickness varied 2nt/h. For wavelength regions in which across each film, and the maximum thick- such a film, deposited on a substrate of ness varied from film to film. A compara- thickness t~, is absorbing, the normal-inci- tively thick film, with maximum thickness dence transmittance is about 20/zm, was deposited in about 3 days T = (I - R)e-~tse -4~kt/x, (2) of sparking.